<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
          *{
            margin: 0;
            padding: 0;
        }
        html,body{
            height: 100vh;
        }
        .content{
height: 100vh;
background-color: red;
        }
        .box1{
            height: 100px;
            background-color: green;
        }
        .box2{
            height: calc(100% - 100px);
            background-color: yellow;
        }
    </style>
</head>
<body>
     <div class="content">
        <div class="box1"></div>
        <div class="box2"></div>
</body>
</html>